Problem: In chemistry class, the girl to boy ratio is $9$ to $10$. If there are $20$ boys, how many girls are there?
Explanation: A ratio of $9$ girls to $10$ boys means that there is a group of $9$ girls for every group of $10$ boys. If there are $20$ boys, then there are $2$ groups of $10$ boys each. Because we know the ratio is $9:10$ , there must be $2$ groups of $9$ girls each. There is a total of $18$ girls in chemistry class.